1. Field of the Invention
The present invention relates to optical communication systems and, more specifically, to monitoring the performance of such systems.
2. Description of the Related Art
Fiber optic networks are widely used for data transmission in modem communication systems. Due to increasing data traffic volumes, monitoring and management of networks become increasingly important. For example, such monitoring may include a measurement at a particular point in the network of the quality of an optical signal corresponding to one or more optical communication channels. The optical signal may be analyzed for various impairments, e.g., timing jitter, noise level, chromatic dispersion, and the like. Obtained information may then be used to improve the performance of the network, for example, if certain impairments exceed a tolerable level.
FIG. 1 shows a block diagram of a representative fiber-optic network 100 configured to carry optical signals modulated with data from a source node 102 to a destination node 112 via a plurality of intermediate nodes 104. An optical transmission link 120 connects a pair of intermediate nodes 104A and 104B via an optical fiber 106. Link 120 may include one or more optical amplifiers, optical regenerators, and optical performance monitors. In FIG. 1, link 120 is illustratively shown as having one optical amplifier (OA) 108, one regenerator 110, and one optical performance monitor (OPM) 114. Amplifier 108 is located between sections 106A and 106B of fiber 106 and is configured to boost optical signals attenuated by light absorption in section 106A. Regenerator 110 is located before node 104B and is configured to correct optical impairments induced by the preceding network components, e.g., fiber 106 and OA 108. OPM 114 is optically coupled to section 106B and is configured to monitor the quality of optical signals received by regenerator 110.
Depending on the particular implementation, regenerator 110 may be designed to perform reshaping and re-amplification (2R) or reshaping, re-amplification, and retiming (3R) of optical signals. Descriptions of representative prior-art 2R and 3R regenerators can be found in U.S. Pat. Nos. 6,498,671, 6,317,232, and 5,353,146, the teachings of all of which are incorporated herein by reference.
In a typical implementation, OPM 114 operates by converting an optical signal into a corresponding electrical signal and then analyzing the electrical signal for impairments using electrical signal processing methods. However, one disadvantage of such an OPM is that it typically requires high-speed electronics. Furthermore, at relatively high bit rates, e.g., about 40 Gb/s, either the sensitivity or bandwidth of the electronics may be insufficient to accurately and/or cost-effectively measure the impairments. On the other hand, optical methods applied to monitoring optical signals have certain advantages over electrical methods because at least part of the signal processing is accomplished in the optical domain. Descriptions of representative optical-domain-based OPMs can be found in U.S. Pat. Nos. 6,433,901 and 6,396,051, the teachings of both of which are also incorporated herein by reference. However, one problem with such optical-domain-based OPMs is that they often require relatively expensive optical components, such as nonlinear crystals, interferometers, tunable filters, etc.